If you act as a certified, insured contract provider you can work up to the level of trained EMR without a medical director. Red Cross is probably the most popular trainers of first aid responders for the public arena.
Whatever an EMR can do, you can do. Be careful though. You don't want to do anything out of the EMR scope. EMR's can provide up to 6 liters of 02 without a medical director for example. One could also assist with medications, provide first aid, manage C-spine until EMS arrives, provide CPR and use an AED....the usual MFR stuff for the most part. Make certain to purchase your AED from an authorized dealer, not from E-bay. You need a legitimately registered unit if you plan on purchasing one. Purchase insurance!! Pehaps from HPSO or some equal organization. Good Luck.